In theory, twisting together the conductors forming a twisted-wire cable, as used in a telephone subscriber loop, ensures that the impedance is balanced throughout its length. In practice, however, there are imbalances. For example, moisture ingress might cause one conductor to have greater leakage to ground, the twisting might not be uniform, and the conductors might be untwisted where taps are made. When a signal propagates along the cable, the waves on the respective conductors encounter different complex impedances. As a result, they may propagate at different speeds and be subject to different distortion. Upon arrival at the receiver, the two waves are no longer symmetrical. Although this effect can usually be ignored in conventional telephone systems, it presents problems in high speed digital subscriber loops, especially Very High speed Digital Subscriber Loops (VDSL) which operate at radio frequencies. Radio-frequency (RF) signals from commercial AM or amateur radio transmitters frequently couple to twisted wire cables, particularly to overhead service drops, as a common mode noise signal. Because of the above-described cable imbalance, some portion of such an RF interference signal will usually convert to differential mode and be coupled inductively across the hybrid device. In addition, the stray capacitance between the input and output of the hybrid will couple the RF interference signal to appear at the output of the hybrid device. This may be significant if the RF interference signal is relatively large in amplitude.
Various systems have been proposed to cancel common mode noise in subscriber loops. In T1E1.4/96-084 dated Apr. 18, 1996, and at a VDSL workshop at IEEE Globecom, Nov. 18, 1996 in London, England, John Cioffi and John Bingham proposed doing so by extracting a signal representing common mode noise and filtering it using an adaptive analog wide band filter to provide a radio frequency noise estimate for subtraction from the differential signal obtained from the secondary of the hybrid transformer. The coefficients of the adaptive filter were tuned during quiet periods, i.e. when no signal is being transmitted, to reduce the difference between the differential signal and the noise estimate signal substantially to zero. Unfortunately, in normal operation, the adaptive analog filter cannot readily compensate for the cross-coupling of the differential signal and common mode signals due to the loop imbalance and so, during signal transmission, will cancel part of the differential information signal too. Accordingly, the resulting signal supplied to the receiver will be distorted.
Canadian patent application No. 2,237,460 filed May 13, 1998, naming one of the present inventors, disclosed a noise suppression circuit in which a narrowband noise detection and control unit scanned the operating band to identify noisy frequency bands and suppressed the noise in those bands. The circuit is not entirely satisfactory because it requires the number of interfering RF signals to be few and does not cancel impulse noise. International patent application No, WO 99/63675 published Dec. 9, 1999, also naming such inventor, disclosed a wideband common mode noise canceller in which a digital common mode signal was filtered by an analysis filter bank to produce subband signals at different frequencies. Previous samples of each of the subband signals were summed and compared with a predetermined noise threshold. If the summed noise signal was greater than the threshold, the subband signal was processed by a synthesis filter to form a component of a noise estimate signal for subtraction from the differential signal. While this circuit will compensate to some extent for stray capacitance of the hybrid device and the above-described loop imbalance caused by inductive coupling close to the receiver, its performance is limited since RF interference often is coupled into the loop as positions far from the receiver.